it would be quite nice if this sort of information could be included in
a Raid-Tuning FAQ or somesuch thing.
-stephen
Gadi Oxman wrote:
>
> On Wed, 18 Aug 1999, Gadi Oxman wrote:
>
> > I'd recommend verifying if the following changes affect the s/w
> > raid-5 performance:
> >
> > 1. A kernel compiled with HZ=1024 instead of HZ=100 -- this
> > will decrease the latency between "i/o submitted to the raid
> > layer" and "i/o submitted to the low level drivers" by allowing
> > the raid-5 kernel thread to run more often.
> >
> > 2. Increased NR_STRIPES constant in drivers/block/raid5.c from 128
> > to 256 of 512; this will potentially queue a larger amount of data
> > to the low level drivers simultaneously.
>
> Another thing which might hurt performance is the hash table scanning
> order in the raid-5 kernel thread.
>
> In the default setup, the hash table can contain up to 1024 entries,
> and the hash function is:
>
> #define stripe_hash(conf, sect, size) ((conf)->stripe_hashtbl[((sect) /
> (size >> 9)) & HASH_MASK])
>
> So that sectors 0 - 1023, 1024 - 2047, etc, will fill the slots 0 - 1023
> in that order (for 512 bytes block size).
>
> Only NR_STRIPES active stripes might be in the hash table at a time,
> and in addition to using the hash table to find a stripe quickly, we
> are also queueing the stripes to the low level drivers by scanning the
> table in increasing order, starting from slot 0.
>
> This means that if, for example, we currently have a 128 pending
> write stripes which wrap around the table, for example for sectors
> 950 - 1077, we will actually queue sectors 1024 - 1077 first, and
> only then queue sectors 950 - 1023, which might be one of the
> causes for sub-optimal performance.
>
> The following patch tries to find the current minimum sector,
> and start running on the table from there in a circular manner,
> so that in the above example, we will queue the sectors in
> increasing order.
>
> Gadi
>
> --- drivers/block/raid5.c~ Fri Jun 18 10:18:07 1999
> +++ drivers/block/raid5.c Wed Aug 18 22:39:06 1999
> @@ -1322,7 +1322,8 @@
> struct stripe_head *sh;
> raid5_conf_t *conf = data;
> mddev_t *mddev = conf->mddev;
> - int i, handled = 0, unplug = 0;
> + int i, handled = 0, unplug = 0, min_index = 0;
> + unsigned long min_sector = 0;
> unsigned long flags;
>
> PRINTK(("+++ raid5d active\n"));
> @@ -1332,8 +1333,22 @@
> md_update_sb(mddev);
> }
> for (i = 0; i < NR_HASH; i++) {
> -repeat:
> sh = conf->stripe_hashtbl[i];
> + if (!sh || sh->phase == PHASE_COMPLETE || sh->nr_pending)
> + continue;
> + if (!min_sector) {
> + min_sector = sh->sector;
> + min_index = i;
> + continue;
> + }
> + if (sh->sector < min_sector) {
> + min_sector = sh->sector;
> + min_index = i;
> + }
> + }
> + for (i = 0; i < NR_HASH; i++) {
> +repeat:
> + sh = conf->stripe_hashtbl[(i + min_index) & HASH_MASK];
> for (; sh; sh = sh->hash_next) {
> if (sh->raid_conf != conf)
> continue;
